Ideally (at least from my perspective), a git version of mesa would be available in experimental repo and frequently updated. I see that some of the necessary pieces (like libdrm/kms) have an updated version in experimental repo, but I'm guessing that's because unstable will be moving to the mesa 9.0.x release soon.

If git mesa is not in experimental repo, and I want to compile it myself, what's the best way to install it? Should I try to make .debs? Should I just put mesa packages on hold and install over them? Should I install to /usr/local and try to point everything there?

The first question is usually: Why? Just because is usually not a good answer to it.

I compiled the whole mesa, drm and X stack for a while (at least the parts concerning intel) from git to test if newer versions would cope better with my hardware (they did), but I was really happy then upstream released these bits and they were uploaded as this wastes quiet a bit of (CPU) time.

What I usually do for anything I need from upstream is get it out of the version control system (git, bzr, hg, svn, …), add the packaging stuff (available via apt-get source $package --diff-only) and run dpkg-buildpackage. Most of the time this fails as upstream did something to their buildsystem, has renamed files, applied debian-patches or whatever. Fix those and at some point you will get *.deb files out of it. Comparing those to the "older" official ones doesn't hurt. If you came that far you should have reasonably good packages with a good chance that they can be installed, removed and/or upgraded later on.

Installing stuff with 'make install' might be faster, but is usually not reversible and it's likely that you will need the debian-patches anyway.

Do all this (or at least the install bit) in a virtual machine first if you are scared (you should be).

As said, it is usually quiet a bit of work, mostly not worth it and requires a fair bit of knowledge if you don't want to reinstall your system every now and then (you basically need the knowledge of a package maintainer – and low-level packages tend to require more knowledge than some random game or something). For self-compiler-people distros like Gentoo and Arch tend to be better. Not that it would be necessarily easier with those, but more people do it as it is the concept of those distros, so more people are able to help.

_________________MfG. DonKult
"I never make stupid mistakes. Only very, very clever ones." ~ The Doctor